(1) Field of the Invention
The present invention relates to an imaging unit detachable from an image forming apparatus, and in particular to a technique for cleaning a positioning member for defining a gap between a photosensitive drum and a developing roller.
(2) Description of Related Art
In electrophotographic image forming apparatuses, the gap between a photosensitive drum and a developing roller (referred to below as “developing gap”) is a factor of great importance that affects an image quality, such as the density and blots due to carrier adhesion. As a method for retaining a prescribed developing gap, it is widely known to provide abutting members, such as gap rollers, on both ends of the developing roller, and bring the abutting members into abutting contact against the surface of the photosensitive drum.
However, with the repeated use of the image forming apparatus, more and more toner adheres to the surface of the photosensitive drum. Such toner present between the photosensitive drum and the gap roller hinders the developing gap from being properly retained, thus causing a risk of degradation of the image quality.
In order to address the above problem, the Japanese patent application publication No. 2003-307931 (referred to below as Document 1) discloses the following structure. That is to say, a cleaner, such as a felt, is provided to be in abutting contact with the surface of the photosensitive drum. The position on the surface of the photosensitive drum at which the cleaner abuts against is located upstream in a rotation direction of the photosensitive drum relative to the position on the surface of the photosensitive drum at which the gap roller abuts against. As the photosensitive drum rotates, the cleaner cleans the abutting portion of the surface of the photosensitive drum.
However, since the cleaning is achieved by the rotation of the photosensitive drum in the structure of Document 1, the technique is not applicable to apparatuses having the abutting members for positioning, including the photosensitive drum, that do not rotate. Furthermore, due to the abutting contact of the cleaner, the rotational loads of the photosensitive drum increases. This brings about a need for using a larger size motor, thereby increasing the cost. Moreover, the cleaner might abrade the surface of the photosensitive drum as a result of abrading, thereby causing a change in the developing gap over time.